The present invention relates to a method of operating a variable speed drive system comprising an electric motor. In another aspect, the present invention relates to a variable speed drive system comprising an electric motor. In still other aspects, the invention relates to a method for refrigerating a hydrocarbon stream using a variable speed drive system or method of operating the same.
An example of a hydrocarbon feed stream that in the industry often requires to be refrigerated is natural gas, for instance in order to produce liquefied natural gas (LNG). Natural gas is a useful fuel source, as well as a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in an LNG plant at or near the source of a natural gas stream. LNG can be suitably stored and transported in bulk form.
A large number of LNG plants comprise one or more compressors in a refrigeration cycle wherein a refrigerant fluid is compressed as it is cycled in a refrigerant circuit. These compressors are loads that are traditionally directly driven by prime movers such as a steam turbine and/or a gas turbine, in some cases assisted by an electric motor as helper drive to assist the turbines and to start the cycle up. However, there is a growing interest in all-electric (exclusively electrically driven) mechanical refrigeration cycles, where the compressors are shaft-driven by a prime mover in the form of an electric motor only. Such motor may form part of a variable speed drive system.
It is a known phenomenon that shaft assemblies in strings of prime movers and loads may exhibit weakly dampened mechanical resonances. These mechanical resonances are often referred to as “torsional modes” or “torsional resonances”. The frequency of such a torsional resonance may fall below the network synchronous frequency. The torque associated with a torsional resonance vibration can be large enough to cause mechanical damage. It is also a known phenomenon that such torsional resonance vibrations can be excited as a result of inter-harmonic current components that are generated in the power electronics used to power the electric motor.
Various solutions have been proposed in the past to address the risk of torsional resonance vibrations. Amongst these, U.S. Pat. No. 7,173,399 discloses an integrated torsional mode dampening system comprising a load commutated inverter system comprising a rectifier, a DC link and a load inverter; a sensor configured for sensing a signal representative of torque on a load side shaft; and an inverter controller configured for using the sensed signal for detecting the presence of a torsional vibration on the load side shaft and generating inverter control signals for damping the torsional vibration by modulating active power through the load inverter.
A drawback of the system disclosed on this US patent is that it needs a sensor for sensing torque on the load side shaft.